The present invention relates to extrusion dies and methods used in forming dual wall corrugated plastic pipe and dual wall plastic pipe having a foam annular core. Dual wall plastic pipe products are utilized typically for drainage and sanitary sewer applications. See, for example, my co-pending applications for Letters patent, Ser. No. 09/579,782, “Mold Block and Methods for Thermoforming Large Diameter Corrugated Plastic Pipe” filed on May 26, 2000, now U.S. Pat. No. 6,652,258 and Ser. No. 09/597,338, “Joint and Seal for Large Diameter Corrugated Plastic Pipe and Method for Joining Plastic Pipe Sections” filed on Jun. 16, 2000, now abandoned. This invention is directed to extrusion dies having axially displaced orifices that define the size, quality and thickness of the inner and outer walls of dual wall plastic pipe products and dual wall plastic pipe products having a foam core.
This invention discloses apparatus and methods for enhancing melt homogeneity and remote real time continuous die lip adjustments for process control and “on the fly” changes of pipe characteristics.